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Geologic Time/Transcript
Transcript Text reads: The Mysteries of Life with Tim and Moby A boy, Tim, and a robot, Moby, are hiking in a desert. Moby stops and points. MOBY: Beep! Tim looks in the direction Moby is pointing. He sees a fossil. TIM: Oh, cool. A fossil! Tim reads from a typed letter. TIM: Dear Tim and Moby, Why do scientists name time periods, like Jurassic and Precambrian? From, Lila. An image shows Earth from space. TIM: Well, kind of to keep track of things. Planet Earth is extremely old, about 4.6 billion years. TIM:Lots of changes have happened over that time. Images show an ocean, a map of the continents, a mountain range, and a headstone for a pterodactyl. TIM: Oceans have formed, continents have moved, mountains have risen, animals have evolved and gone extinct. To organize major changes, scientists have broken the Earth's timeline into sections lasting millions of years. This is called the geologic time scale. A timeline shows Earth's geologic history over billions of years. MOBY: Beep. TIM: No. Instead of millennia, centuries, and years, the geologic time scale is broken up into eons, eras, periods, and epochs. The image of the geologic time scale is divided into eons, eras, periods, and epochs.. TIM: The Earth has had four eons, each lasting hundreds of millions to billions of years. Each of these eons is broken up into two or more eras, which are broken into periods, which are broken into epochs. All these units are of relative length, so they have no set number of years. The actual length of each unit depends on what geologic changes occurred in it. Like, one eon could contain a single era that's longer than another. For example, the Devonian period, when amphibians first evolved, lasted about fifty to fifty-five million years. A timeline shows the length of the Devonian period. An amphibian fossil appears above it. TIM: While the Silurian period, which saw the first jawed fish, lasted fewer than thirty million years. The shorter Silurian period is shown on the timeline. A jawed fish appears above it. MOBY: Beep. TIM: Those names may sound confusing, but they usually have a pretty simple explanation. The Devonian period, for example, is named after Devon, England, where rock layers from that time were first studied. A map shows Devon, England. TIM:The basis for the entire timeline is a branch of geology called stratigraphy. That's the study of rock layers, and the fossils contained inside them. An image shows rock layers, each with various plant and animal fossils. Progressively more complex animals appear in the higher layers. TIM: According to stratigraphy, a rock layer, or stratum, is probably older than the layers above it and younger than the layers below it. So, by studying the different layers, scientists can get a good idea of how animals evolved and how the rock composition of an area has changed over time. MOBY: Beep. TIM: Right, and they can also put everything into an order. An image shows fossils being arranged on the geologic timeline. TIM: Stratigraphy isn't perfect, though. Over millions of years, rock can break, bend, erode, and shift. An image shows a scientist with a clipboard examining geological core samples from different parts of the world. TIM: To get a more complete picture of conditions on our planet, scientists compare the strata from one area of the Earth to another. They also employ radiometric dating, which uses radioactive isotopes to date objects. Images show the geologic timeline and a scientist in front of a computer using radiometric dating to date a fossil. TIM: As opposed to the geologic timeline's relative dates, radiometric dating gives an absolute date. MOBY: Beep. TIM: Right now? Well, we're living in the Holocene epoch of the Quaternary period of the Cenozoic era of the Phanerozoic eon. Tim points. TIM: Hey, look at that. Tim and Moby look at an ancient wall painting of a spaceship and a figure who looks like Moby. TIM: Weird. MOBY: Beep. Moby taps Tim on the head and makes an electrical sound. Tim becomes glassy-eyed, as if hypnotized. TIM: Whoa. I. Am. Not. Interested. In. That. Painting.Category:BrainPOP Transcripts